Changes in fragmentation of western Washington forest land
Andrew Gray1, Tyler Robinson1, and Beth Willhite2
1) USDA Forest Service PNW Research Station FIA, Corvallis OR; 2) USDA Forest Service Region 6 FHP, Sandy OR
WWA change in non-NFS
timberland area
1
Introduction
FIA monitoring has detected very rapid conversion of forest
land to nonforest land uses in western Washington in the last
decade, with 5% of private timberland (270,000 acres) being
converted between 1990-2000 (Fig. 1), following a similar rate
of loss in the previous decade (Fig. 2). The human population
in the state has almost doubled since 1970 (Figs 3a + 3b). It
is likely, but unknown, whether increased fragmentation of
forest land has also occurred. Fragmentation reduces patch
size and increases edge environments, with important effects
on fire risk, weedy species, timber management, and wildlife
habitat. The objective of this project was to evaluate the extent
and spatial pattern of forest fragmentation in western
Washington and begin to assess its implications.
Loss of forestland
3a
Timberland 1990
Net to NFS
7,397
-30 Snoqualmie Pass, Gorge
To reserved
To non-forest
-60 Primarily DNR
-270 4.7% of private area
Timberland 2000
7,037
Net private to public
148
(Gray et al. 2005. PNW-RB-246)
WWA: loss of private timberland
2
1.4 million acres lost 1935-2000, some to public, ~25,000 ac/yr to nonforest
6.0
Population density:
People / 1,000 acres
2.5 - 25
Population=3,413,000
5.0
26 - 50
501 - 1000
1001 - 1366
Million acres
51 - 100
101 - 500
4.0
3.0
2.0
1.0
0.0
1935
1980
1990
2000
2010?
Year
3b
Census: 2007 Population density
Population density:
People / 1,000 acres
2.5 - 25
Population=6,468,000
26 - 50
51 - 100
101 - 500
501 - 1000
1001 - 1366
Forest fragmentation
(thousand acres)
7.0
Census: 1970 Population density
5a
Methods
Imagery from three time periods was acquired—mid-1970s, early
1990s, and 2006—and land cover is being manually classified into
GIS polygons of different land use zones for each time period:
Wildland forest: area >=640 ac, <5 dispersed developments per
640 ac, and >80% forest land
Intensive agriculture: area >=640 ac, <9 dispersed developments
per 640 ac, and >80% agricultural land.
Mixed forest/agriculture: area >=640 ac; <9 dispersed
developments per 640 ac; and mixed forest, range, and agriculture
with forest >= 50% of the non-agricultural land.
Low-density residential: area any size with >=9 developments per
640 ac
Change in land use varied
dramatically by county, with
the greatest changes
occurring in Clark County,
near the Portland metro
area (Figs 5a + 5b).
Between 1976 and 2006,
forest, agricultural, and
mixed land in Clark county
declined by 11, 32, and
25%, while low-density
residential and urban land
increased by 60 and 124%,
respectively. In
comparison, forestland loss
in Pacific and Wahkiakum
counties was 1%. For
southwest Washington as a
whole, most of the
developing low-density
residential areas between
1976 and 2006 came from
forest land, while most of
the developing urban areas
came from agricultural and
low-density residential land
(Fig 6).
6
Forest fragmentation increased in all ecoregions in
southwest Washington, as indicated by decreases in areato-perimeter ratios and mean patch size between 1976 and
2006 (Fig. 8).
8
5b
13
2006 Area:
0
-2,332
-1,824
-61,463
-2,828
-5,074
-73,520
2,332
0
456
-15,511
-5,467
0
-18,192
1,824
-456
0
-12,760
-16,870
0
-28,261
61,463
15,511
12,760
0
-15,624
-482
73,628
2,828
5,467
16,870
15,624
0
0
40,788
5,074
0
0
482
0
0
5,556
2,573,526
103,618
201,876
221,306
90,714
6,472
4
Houses in the woods
7
45
40
1976
1994
2006
Clark County
2006
994,953
2,557
389
82,913
Coast
Coast
Coast
Coast
Ranges
Ranges
Ranges
Ranges
Total area (ha)
Total perimeter (km)
Area/Perimeter (ha/km)
Mean area (ha)
430,001
1,998
215
25,294
427,090
2,016
212
22,478
425,522
2,052
207
22,396
Puget
Puget
Puget
Puget
+ Willamette lowlands
+ Willamette lowlands
+ Willamette lowlands
+ Willamette lowlands
Total area (ha)
Total perimeter (km)
Area/Perimeter (ha/km)
Mean area (ha)
145,318
2,884
50
10,380
138,996
2,980
47
8,687
133,648
3,205
42
5,346
Total area (ha)
Total perimeter (km)
Area/Perimeter (ha/km)
Mean area (ha)
1,590,400
6,464
246
38,790
1,568,087
6,902
227
34,846
1,554,124
7,291
213
27,752
All SWWA
All SWWA
All SWWA
All SWWA
Structures (#/mi2)
The number of structures on
forest land increased in all
southwest counties, ranging
from 14 to 72% increase by
county, and 56% overall (Fig.
7). In Clark County, 29% of the
points that were within ½ mile of
low-density residential or urban
in 1976 changed to low-density
residential or urban by 2006
(only 3% of points outside ½
mile changed). Forest that is
within ½ mile of low-density
residential or urban also has
significantly more structures,
averaging 3.6 and 7.0
structures/square mile within ½
mile in 1976 and 2006
respectively. Outside ½ mile
these averages were 1.8 and
2.6.
Market Value of Land Per Acre by Distance to Urban
7000
6000
5000
4000
3000
2000
1000
0
30
25
Urban
20
15
<1/4 Mile 1/4 to 1/2 1/2 to 3/4 3/4 to 1
Mile
Mile
Mile
>1 Mile
* Parcel data provided by Luke Rogers, University of Washington
10
5
0
Wildland Forest
Mixed Forest/Ag
Intensive Agriculture
40
35
1976
1994
2006
11
Conclusions
Pacific County
Results to date suggest that
forest fragmentation is
increasing in southwest
Washington; continuing work
(e.g. Fig. 11) will quantify
patterns across the state.
30
25
20
15
10
5
0
Wildland Forest
Mixed Forest/Ag
Intensive Agriculture
40
35
1976
1994
2006
Southwest Washington
30
25
20
15
10
5
Wildland forest and agriculture land uses, Cowlitz
River valley, Lewis County
1994
1,002,000
2,462
407
100,200
10
35
Structures (#/mi2)
New houses outside Sequim, Olympic Peninsula
We are also assessing changes in tree cover in urban zones. To
date we have completed all three time periods for southwest
Washington (Fig. 4), and are about half-way through the work in
northwest Washington.
1976
1,015,081
2,167
468
101,508
Similar amounts of development
have occurred in both Oregon
and Washington in the Portland
area (Fig. 9). However,
development is less dispersed in
Oregon, indicating that land-use
laws in place since the 1970s
have been effective. Preliminary
analysis of land value in King
county indicates higher assessed
value of land close to urban
areas (Fig. 10), which may
indicate pressure to develop
those lands over other uses.
Low-density
NonForest
Mixed
Agriculture residential Urban
vegetated
2,647,047
121,810
230,137
147,677
49,926
916
Structures (#/ac)
In addition, 44,500 systematically-placed points were identified
outside federal lands across the state and man-made structures
(e.g., single houses, or houses with barns) in an 80 ac circle around
each point are being counted in non-urban land-use zones. Distance
to the nearest contrasting land-use is calculated for each point.
Metric
Total area (ha)
Total perimeter (km)
Area/Perimeter (ha/km)
Mean area (ha)
9
Urban: area >=40 ac; commercial, service, and residential uses with
a city road pattern
Non-vegetated: area >=640 ac covered with water, ice, rock, or
sand.
Ecoregion Section
Cascades
Cascades
Cascades
Cascades
Policy implications
Net changes in area by land use, nonfederal land, 1976 to 2006 (acres)
Change in:
1976 Area:
Change to/from:
Forest
Mixed
Agriculture
Low-density residential
Urban
Nonvegetated
Net Change
Year
0
Wildland Forest
Mixed Forest/Ag
Intensive Agriculture
Funding for this project was provided by
the Forest Health Monitoring program and
the PNW Research Station